Poly(vinyl alcohol)−Borate Hydro/Cosolvent Gels: Viscoelastic Properties, Solubilizing Power, and Application to Art Conservation

Abstract

We report the development of a new type of hydrogel in which a cosolvent has been added to the water component. The gel networks are based on the well-known poly(vinyl alcohol)-borate systems (PVA-borate). However, it is shown that the rheological and solubilizing properties of the hydrogels can be modified drastically by the addition of a cosolvent. The studies have focused on 1-propanol as the added liquid, although it is shown that others (propylene carbonate, 1-pentanol, cyclohexanone, and 2-butanol) are amenable to making modified hydrogels as well. In addition to the rheological measurements, the gels have been investigated by differential scanning calorimetry (free water index) and determination of their solubilizing power. Finally, the gels have been applied to clean and oxidized varnish (patina) from the surface of a XVI-XVII century oil-on-wood painting by Ludovico Cardi detto il Cigoli. The mode of cleaning by and removal of the PVA-borate water/1-propanol gel from the painted surface demonstrate several advantages over other gels used in art conservation.

Poly(vinylalcohol)-borate hydrogels with improved features for the cleaning of cultural heritage surfaces

“…The most evident difference is the drastic decrease of FWI in the formulations including the organic solvent, in strict agreement with results reported in the literature [14,18,19,25]. In the concentration range here investigated,…”

“…The state of water in polymeric dispersion may be determined through the use of the Free Water Index. The melting enthalpy of water is derived from DSC thermograms, by integration of the melting endothermic peak and is used to calculate the FWI from the following equation [18]:…”

“…These preparations, based on poly(acrylic acid) as a gelator, minimize the negative effects related to the penetration of liquid, but some residues can remain on the surface after the cleaning process [17]. PVA/borax hydrogels, on the other hand, show mechanical features suitable for easy removal by peeling from surfaces, without need of strong mechanical or chemical actions, and have been therefore proposed for cleaning purposes [14,[18][19][20]. Water/ cosolvent mixtures can be embedded into these high viscosity polymeric dispersions, making them able to soften and swell water-insoluble materials, in a way similar to that proposed in a study concerning the solubilization of hydrophobic substances by hydrotropes in water [21].…”

“…The presence of water miscible organic solvents in the PVA/borax gels has been shown to strongly modify the gel physical properties [18,20], and when the application to cleaning of artistic surfaces is concerned, the main interest is to obtain controlled rheological properties and to expand the compatibility with organic solvents, starting from known formulations and modifying them by the addition of suitable additives. In the present study we have investigated the effect of adding poly(ethyleneoxide) (PEO) in various concentrations on the mechanical properties of PVA/borax gels and its influence on the capability of retaining organic solvents.…”

Self-consistent field theory of polymer-ionic molecule complexation

“…The reversibility of the complexation processes can be utilized for various potential applications in chemical engineering or supramolecular science to fabricate functional materials such as hydrogel. 21 Adsorption phenomena are often described by the Langmuir model, where an adsorbate molecule ͑B͒ is adsorbed onto an unoccupied binding site ͑P͒ to form an adsorbed complex ͑BP͒. The Langmuir model is based on the following assumptions: ͑1͒ the adjacent molecules adsorbed on the binding sites are mutually noninteracting, ͑2͒ the surface at which there are binding sites is geometrically uniform ͑i.e., not corrugated͒, and ͑3͒ each binding site is capable of adsorbing a single molecule.…”